Diamond Ring at Second Contact - 2017 Total Solar Eclipse

In the final seconds before the total eclipse begins, the shrinking crescent Sun produces a short, brilliant segment along one edge of the Moon (i.e., the diamond) as the solar corona (i.e., the ring) emerges into view in the rapidly darkening sky.
The 'diamond' grows smaller and breaks up into a string of bright points (Baily's Beads (Wikipedia)) that vanish as totality begins.